Newly emerging battery requirements such as are to be found in the Strategic Defense Initiative program and in battery-powered automobiles have recently focused considerable effort into the development of battery systems having larger energy and power density capabilities. The need for higher energy density has in particular urged battery development efforts toward ultra-thin cathode electrode assemblies. In the thin electrode art, however, it is found that one of the more difficult components to produce with small physical size and desirable consistency, accuracy, and low cost is a finely-divided particle or particle composite electrode structure. In such structures, finely-divided particulate matter such as carbon black, powdered nickel or other large surface area generating substances are however, needed for providing a desirable low electrical resistance interface with the battery electrolyte and for other benefits. Finely divided materials of this type have heretofore been somewhat difficult to control with respect to their layer thickness and region-to-region uniformity, for example, in an electrode assembly.
As is generally known in the electrical battery art, one class of useful arrangement for a battery cell involves the lithium thionyl chloride and lithium gallium chloride systems in which a liquid electrolyte enters into a chemical reaction at both the surface of a lithium anodic electrode element and also at the surface of a catalytic cathode electrode element. Moreover, cells of this type are known to provide a desirable and relatively high energy density battery arrangement.
In the cathode electrode for cells of this type, a catalytic material such as finely-divided carbon is disposed on an electrode substrate member where it provides the large surface areas that are desirable both for electrical conduction to the liquid electrolyte reactant materials and for locating reaction sites for the reactant materials. Parenthetically, it is notable that the useful life of finely-divided material cathode electrodes usually ends when the deposition of solid reactant products into the space between adjacent carbon particles and over the surface area of the carbon particles has proceeded until the effective surface area of the carbon is severely reduced. For this reason it is desirable to both provide a somewhat large amount of the carbon particles or other particulate material and to dispose this material on the electrode surface in a form which provides maximum particle accessibility to the liquid electrolyte--that is, in a form which is acceptable without use of the mechanical compaction which has been commonly used in the formation of such electrodes. Such disposition of the electrode particulate material is, however, in conflict with the need for small physical size in the electrode and high energy density performance of the electrolytic cell.
The patent art indicates a number of attempts to improve upon the arrangement of fine particle electrode structures; these attempts are evidenced by the patents cited below. It is significant to note, however, that notwithstanding these prior patent improvements, there has heretofore been a notable absence of an electrode arrangement wherein satisfactory uniformity of the small particle material and an electrode thickness that is small enough to be compatible with the high energy density and high power density needed for present-day state-of-the-art applications has been available.
The prior patent of one present invention co-inventor, Franz Goebel, U.S. Pat. No. 4,118,334, for example, describes a primary battery or electrochemical cell which employs a carbon black inclusive cathode element. The carbon black material of the '334 patent is prepared by a dried slurry crumbling process which results in the formation of cathode material globules. Each of the globules has minute pores for achieving electrolytic solution contact. The drying and crumbling to achieve globules of cathode material are, however, notable points of distinction between the present invention and teachings of the '334 patent.
Another physical arrangement and its associated processing of the carbon black material for a cathode structure is described in the patent of J. E. Barnes et al, U.S. Pat. No. 4,296, 187. wherein a wet carbon slurry material which includes a mixture of carbon black, water, and/or isopropyl alcohol together with a binder such as finely-divided TEFLON.RTM. is rolled into a layer of predetermined thickness following its deposition onto a porous fiberglass substrate. The '187 patent also employs porous carbon globules, however, these globules are subjected to a plurality of rolling compaction steps. The carbon processing sequence including these rolling steps distinguishes the cathode processing of the present invention from the '187 patent.
Yet another electrochemical cell cathode arrangement which involves carbon black material is described in the patent of K. A. Klinedinst et al, U.S. Pat. No. 4,352,866, in which spraying of a liquid dispersion onto a substrate in a thin cathode structure is described. The '866 patent also involves a bonding material which may be a thermoplastic polymer, the use of water, ethanol, and isopropanol and an ultrasonic mixing of the dispersion components. The goal of achieving a thin cathode structure is pursued in several examples in the '866 patent, including example IV, wherein alternate spraying and drying onto a metallic foil such as a nickel foil of 127 micrometers thickness is described. The use of ultrasonic energy for the limited purpose of mixing liquid dispersion components is a principal distinction between teachings of the '866 patent and applicants' invention.
Another arrangement for manufacturing porous carbon structures is disclosed in the additional patent of Franz Goebel et al, U.S. Pat. No. 4,562,094, wherein a slurry of finely-divided carbon material is deposited on a substrate of stainless steel sheet, nickel screen, or woven or unwoven fiberglass materials with the slurry including a FREON (TF).RTM. or trichlorotrifluoroethane liquid. The '094 patent also utilizes a tetrafluoroethylene binder material and additionally includes a summary of the above-described Barnes et al patent. The present invention is distinguished over the '094 patent by the differing liquid vehicle material and the differing dispersion of slurry materials.
Despite the use of conventional spraying, thin foil substrates, tetrafluoroethylene binder materials, ultrasonic agitation of slurry component mixtures, multiple layered small particle films and other tangential similarities to the present invention in the above-described patents, the prior art has heretofore lacked an arrangement which fully achieves the processing and performance benefits of the present invention.